Relative Success Of Sequence Stratigraphy In Exploration Indicated By Drilling Statistics

Abstract

ABSTRACT The application of sequence stratigraphic concepts, first Proposed during the mid- 1970's, has led to both successes and failures in petroleum exploration. Although a broad spectrum of reservoir occurrences are predicted in siliciclastic sequence stratigraphic models, the most successful applications have been in incised valley-fills (Iowstand and transgressive systems tracts) and turbidite systems (lowstand systems tract). We analyze one example from each of these depositional settings which helps point out the successes and limitations of sequence stratigraphy in exploration. The first example is from the Pennsylvanian Morrow Formation of southeastern Colorado. This is an incised valley-fill trend that produces from several different valley-1511 systems. Of the 181 wells drilled in Clifford to Siaana fields trend, 41.4% of the wells penetrated the valley (75 wells), 19.3% of the wells found reservoir quality sandstone (35 wells), and 9. new fields were discovered (5% rate of success). Since 1985, when the play was interpreted as an incised valley-fill, drilling statistics indicate that there was no increase in the success rate of discoveries, even though a greater percentage of wells penetrated valley-fill facies. This case study indicates that the major problems in valley-fill exploration were 1) locating the valley, 2) locating reservoir sandstone within the valley, and 3) finding a trap for the reservoir. Exploration by Soekor (Ltd.) Pty in the Lower Cretaceous turbidite systems of the Bredasdorp basin in offshore South Africa is ideal for demonstrating the frontier exploration applications of sequence stratigraphy. Basin-floor fans and basin-floor channel fill targets had 30% (14 wells) success ratio, whereas slope fan and prograding complexes had only one success. Dri/ling in the adjacent Pletmos basin had no success in five wells that targeted low stand turbidite systems. These drilling results indicate that sequence stratigraphy helps in exploration by defining play boundaries and prospects. However, they suggest that there may not be an increase in the individual success ratio on prospects. INTRODUCTION Stratigraphic concepts have evolved considerably since the publication of AAPG Memoir 26 on seismic stratigraphy1. As seismic stratigraphy evolved into sequence stratigraphy, the geologic literature has become replete with papers addressing its various aspects. The emphasis in process sedimentology, facies associations, and genetic stratigraphy in the geologic literature of the 1970's and 1980's has educed more encompassing sequence stratigraphic interpretations, which also emphasize the significance of key surfaces. Proponents of sequence stratigraphy assert that its application enhances exploration and production by providing interpretations to better delineate fields and reservoir compartments. The challenge for petroleum geologists is how to use these concepts to find more petroleum either through exploration or field development, This paper reviews two well-documented examples where siliciclastic sequence stratigraphic concepts have been applied in exploration since the mid-1 970's. The main exploration trends that will be discussed are the valley-fill reservoirs of the Pennsylvanian Morrow Formation of southeastern Colorado, and the turbidite systems of the Lower Cretaceous strata of offshore South Africa.